IGCSE Chemistry: Experimental Techniques and Chemical Analysis Practice Questions
Chemical analysis identifies substances using characteristic tests. Gases are identified by simple tests such as the lighted splint and limewater, cations by their reactions with sodium hydroxide and ammonia, and anions by specific precipitation and gas tests.
Topic 12 of Cambridge IGCSE Chemistry 0620 is the most memorisable topic on the syllabus and therefore the cheapest marks available. Every test has a fixed reagent, observation and conclusion. The questions below cover chromatography, gas tests and ion tests in the exact format the papers use.
What you need to know for Experimental Techniques and Chemical Analysis
- Tests for gasesHydrogen gives a squeaky pop with a lighted splint. Oxygen relights a glowing splint. Carbon dioxide turns limewater milky. Ammonia turns damp red litmus paper blue. Chlorine bleaches damp litmus paper.
- Rf valueRf = distance travelled by the spot divided by distance travelled by the solvent front, both measured from the baseline. It has no units and is always less than 1.
- Chromatography rulesThe baseline is drawn in pencil, because ink would dissolve in the solvent and run. The solvent level must start below the baseline, or the spots would dissolve into the solvent directly.
- Separation methodsFiltration separates an insoluble solid from a liquid. Crystallisation obtains a solid from its solution. Simple distillation obtains the solvent from a solution. Fractional distillation separates liquids with different boiling points.
- Tests for anionsCarbonate: add dilute acid, effervescence, and the gas turns limewater milky. Chloride: add dilute nitric acid then silver nitrate, giving a white precipitate. Sulfate: add dilute nitric acid then barium nitrate, giving a white precipitate.
- PurityA pure substance melts and boils at a single sharp temperature. An impurity lowers and broadens the melting point and raises the boiling point.
IGCSE Chemistry Experimental Techniques and Chemical Analysis questions and answers
4 exam-style questions written to the 0620 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
In a chromatography experiment the solvent front travels 8.0 cm from the baseline and a spot travels 3.2 cm. Calculate the Rf value, and explain why the baseline must be drawn in pencil.
Show the worked answer
- Rf = distance travelled by the spot divided by distance travelled by the solvent front.
- Rf = 3.2 divided by 8.0.
- Rf = 0.40. The value has no units because it is a ratio of two distances, and it is always less than 1.
- The baseline is drawn in pencil because pencil is insoluble in the solvent. Ink would dissolve and travel up the paper with the solvent, ruining the chromatogram and the measurements.
Describe a test for hydrogen gas and a test for carbon dioxide gas, giving the observation and conclusion for each.
Show the worked answer
- Test for hydrogen: hold a lighted splint at the mouth of the test tube.
- A squeaky pop is heard, which confirms hydrogen is present. The pop is the hydrogen burning rapidly with oxygen in the air.
- Test for carbon dioxide: bubble the gas through limewater, which is calcium hydroxide solution.
- The limewater turns milky or cloudy white, caused by a precipitate of calcium carbonate. This confirms carbon dioxide.
A colourless solution is thought to contain either sodium chloride or sodium sulfate. Describe a test that would distinguish between them, including the reagents, the observation and the conclusion.
Show the worked answer
- Take a sample of the solution and add dilute nitric acid. This removes any carbonate ions, which would otherwise give a misleading precipitate.
- Add a few drops of barium nitrate solution.
- A white precipitate of barium sulfate confirms that sulfate ions are present, so the solution is sodium sulfate.
- If no precipitate forms, test a fresh sample with dilute nitric acid followed by silver nitrate solution. A white precipitate of silver chloride confirms chloride ions and therefore sodium chloride.
Explain how measuring the melting point can show whether a solid sample is pure.
Show the worked answer
- A pure substance has a fixed melting point and melts sharply, over a range of no more than about one degree.
- Compare the measured melting point with the accepted value from a data book.
- If the sample melts at the accepted temperature and over a very narrow range, it is pure.
- An impurity lowers the melting point and causes the sample to melt over a wider range of temperatures, so a low, broad melting range shows the sample is impure.
Common mistakes in this topic
- Confusing the lighted splint with the glowing splint test.
- Forgetting to acidify before adding silver nitrate or barium nitrate.
- Drawing the chromatography baseline in ink.
- Starting the chromatography with the solvent above the baseline, which dissolves the spots directly into the solvent.
- Giving an Rf value greater than 1, which is impossible.
Exam tips
- Write out every test as reagent, observation, conclusion. Answers structured that way collect every available mark.
- Learn the five gas tests. They appear on almost every paper and take ten minutes to memorise.
- For cation tests, learn which hydroxide precipitates dissolve in excess sodium hydroxide, and which dissolve in excess ammonia.
- Rf values have no units. Writing cm after the answer loses the mark.
- In separation questions, name the technique and justify it from the properties of the substances involved.
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Experimental Techniques and Chemical Analysis FAQs
How do I calculate an Rf value?
Divide the distance travelled by the spot by the distance travelled by the solvent front, measuring both from the baseline to the centre of the spot and to the solvent front respectively. The value has no units and is always less than 1, because the spot cannot travel further than the solvent.
What are the tests for the common gases?
Hydrogen gives a squeaky pop with a lighted splint. Oxygen relights a glowing splint. Carbon dioxide turns limewater milky. Ammonia turns damp red litmus paper blue. Chlorine bleaches damp litmus paper, turning it white.
Why must dilute nitric acid be added before testing for chloride or sulfate?
Carbonate ions in the sample would also form a white precipitate with silver nitrate or barium nitrate, giving a false positive. Adding dilute nitric acid first reacts away any carbonate present, so any precipitate that then forms must be due to the ion being tested for.
How does an impurity affect melting and boiling points?
An impurity lowers the melting point and makes the substance melt over a range of temperatures rather than sharply. It raises the boiling point. A pure substance melts and boils at fixed, sharp temperatures matching the accepted data book values.
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Written to the published Cambridge IGCSE Chemistry (0620) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.